1. Field of the Invention
The present invention relates to a spot welding control method and apparatus using a servo welding gun.
2. Description of Related Art
Automobile body panels are spot-welded using a welding apparatus constructed of a general purpose robot having six degrees of freedom and a spot welding gun coupled to a wrist portion of the robot. The conventional spot welding gun has a pair of electrode tips for pressing a workpiece therebetween, and one of the pair of electrode tips is reciprocally moved by an air cylinder.
Meanwhile, a servo welding gun having a pair of electrode tips, one of which is driven by a servo motor, was proposed by the present applicant in Japanese Patent Application No. 4-94916 filed on Mar. 23, 1992. In the proposed servo welding gun, contact of the welding tip with a workpiece was controlled based on only the servo motor electric current.
However, the proposed spot welding control using the servo welding gun does not fully utilize the capabilities of the servo welding gun, and has the following problems which need to be solved.
First, if some obstacle happens to come between the pair of electrode tips so that the moving electrode tip contacts the obstacle, the servo motor electric current suddenly increases and an erroneous assumption will be made that the electrode tip is in contact with the workpiece.
Second, temperature adhesion of the electrode tip with the workpiece is visually judged by sight by a worker. The servo motor electric current is not used in an automatic detection of such temperature adhesion.
Third, because only one of the pair of electrode tips is moved, changing the contact position where the electrode tip contacts the workpiece will cause a change in the pressing force of the electrode tip. In some cases, the workpiece may be damaged by excessive force from the electrode tip.
Fourth, no measure to counter fused metal dispersion at the welding nugget is taken. More particularly, any signal of the servo motor is used in detecting or predicting generation of fused metal dispersion.
Fifth, electric resistance between the electrode tip and the workpiece is not decreased in the servo welding gun system, though a large electric resistance between the electrode tip and the workpiece tends to generate fused metal dispersion at the nugget.